Molten bath smelting or other pyrometallurgical operations which require interaction between the bath and a source of oxygen-containing gas utilize several different arrangements for the supply of the gas. In general, these operations involve direct injection into molten matte/metal. This may be by bottom blowing tuyeres as in a Bessemer type of furnace or side blowing tuyeres as in a Peirce-Smith type of converter. Alternatively, the injection of gas may be by means of a lance to provide either top blowing or submerged injection. Examples of top blowing lance injection are the KALDO and BOP steel making plants in which pure oxygen is blown from above the bath to produce steel from molten iron. Another example of top blowing lance injection is provided by the smelting and matte converting stages of the Mitsubishi copper process, in which injection lances cause jets of oxygen-containing gas such as air or oxygen-enriched air to impinge on and penetrate the top surface of the bath, respectively to produce and convert copper matte. In the case of submerged lance injection, the lower end of the lance is submerged so that injection occurs within rather than from above a slag layer of the bath, to provide top submerged lancing (TSL) injection, a well known example of which is the Outotec Ausmelt TSL technology which is applied to a wide range of metals processing.
With both forms of injection from above, that is, top blowing and TSL injection, the lance is subjected to intense prevailing bath temperatures. The top blowing in the Mitsubishi copper process uses a number of relatively small steel lances which have an inner pipe of about 50 mm diameter and an outer pipe of about 100 mm diameter. The inner pipe terminates at about the level of the furnace roof, well above the reaction zone. The outer pipe, which is rotatable to prevent it sticking to a water-cooled collar at the furnace roof, extends down into the gas space of the furnace to position its lower end about 500-800 mm above the upper surface of the molten bath. Particulate feed entrained in air is blown through the inner pipe, while oxygen enriched air is blown through the annulus between the pipes. Despite the spacing of the lower end of the outer pipe above the bath surface, and any cooling of the lance by the gases passing through it, the outer pipe burns back by about 400 mm per day. The outer pipe therefore is slowly lowered and, when required, new sections are attached to the top of the outer, consumable pipe.
The lances for TSL injection are much larger than those for top blowing processes such as the Mitsubishi process described above. A TSL lance usually has at least an inner and an outer pipe, as assumed in the following, but may have at least one other pipe concentric with the inner and outer pipes. Typical large scaleTSL lances have an outer pipe diameter of 200 to 500 mm, or larger. Also, the lance is much longer and extends down through the roof of a TSL reactor, which may be about 10 to 15 m tall, so that the lower end of the outer pipe is immersed to a depth of about 300 mm or more in a molten slag phase of the bath. A lower extent of the TSL Lance, including the submerged portion is protected by a coating of solidified or frozen slag formed and maintained on the outer surface of the outer pipe by the cooling action of the injected gas flow. The inner pipe may terminate at about the same level as the outer pipe, or at a higher level of up to about 1000 mm above the lower end of the outer pipe. Thus, it can be the case that the lower end of only the outer pipe is submerged. In any event, a helical vane or other flow shaping device may be mounted on the outer surface of the inner pipe to span the annular space between the inner and outer pipes. The vanes impart a strong swirling action to an air or oxygen-enriched blast along that annulus and serve to enhance the cooling effect as well as ensure that gas is mixed well with fuel and feed material supplied through the inner pipe. The mixing occurs substantially in a mixing chamber defined by the outer pipe, below the lower end of the inner pipe where the inner pipe terminates a sufficient distance above the lower end of the outer pipe.
The outer pipe of the TSL lance wears and burns back at its lower end, but at a rate that is considerably reduced by the protective frozen slag coating than would be the case without the coating. However, this is controlled to a substantial degree by the mode of operation with TSL technology. The mode of operation makes the technology viable despite the lower end of the lance being submerged in the highly reactive and corrosive environment of the molten slag bath. The inner pipe of a TSL lance may be used to supply feed materials, such as concentrate, fluxes and reductant to be injected into a slag layer of the bath, or it may be used for fuel such as fuel oil, particulate coal or communuted plastics material. An oxygen containing gas, such as air or oxygen enriched air, is supplied through the annulus between the pipes. Prior to submerged injection within the slag layer of the bath being commenced, the lance is positioned with its lower end, that is, the lower end of the outer pipe, spaced a suitable distance above the slag surface. Oxygen-containing gas and fuel, such as fuel oil, fine coal or hydrocarbon gas, are supplied to the lance and a resultant oxygen/fuel mixture is fired to generate a flame jet which impinges onto the slag. This causes the slag to splash to form, on the outer lance pipe, the slag layer which is solidified by the gas stream passing through the lance to provide the solid slag coating mentioned above. The lance then is able to be lowered to achieve injection within the slag, with the ongoing passage of oxygen-containing gas through the lance maintaining the lower extent of the lance at a temperature at which the solidified slag coating is maintained and protects the outer pipe.
With a new TSL lance, the relative positions of the lower ends of the outer and inner pipes, that is, the distance the lower end of the inner pipe is set back, if at all, from the lower end of the outer pipe, is an optimum length for a particular pyrometallurgical process operating window determined during the design. The optimum length can be different for different uses of TSL technology. Thus, in a two stage batch operation for converting copper matte to blister copper with oxygen transfer through slag to matte, a continuous single stage operation for converting copper matte to blister copper, a process for reduction of a lead containing slag, or a process for the smelting an iron oxide feed material for the production of pig iron, all have a different respective optimum mixing chamber length. However, in each case, the length of the mixing chamber progressively falls below the optimum for the pyrometallurgical operation as the lower end of the outer pipe slowly wears and burns back. Similarly, if there is zero offset between the ends of the outer and inner pipes, the lower end of the inner pipe can become exposed to the slag, with it also being worn and subjected to burn back. Thus, at intervals, the lower end of at least the outer pipe needs to be cut to provide a clean edge to which is welded a length of pipe of the appropriate diameter, to re-establish the optimum relative positions of the pipe lower ends to optimize smelting conditions.
The rate at which the lower end of the outer pipe wears and burns back varies with the molten bath pyrometallurgical operation being conducted. Factors which determine that rate include feed processing rate, operating temperature, bath fluidity and chemistry, lance flows rates, etc. In some cases the rate of corrosion wear and burn back is relatively high and can be such that in the worst instance several hours operating time can be lost in a day due to the need to interrupt processing to remove a worn lance from operation and replace it with another, whilst the worn lance taken from service is repaired. Such stoppages may occur several times in a day with each stoppage adding to non-processing time. While TSL technology offers significant benefits, including cost savings, over other technologies, the lost operating time for the replacement of lances carries a significant cost penalty.
Our co-pending application PCT/AU2012/000751, filed on 27 Jun. 2012 discloses a new top submerged lance which enables a reduction in time lost through the need for lance replacement for repair. The features of the new lance of application PCT/AU2012/000751 are applicable to a wide range of top submerged lances in enabling adjustment, relative to the lower end of the outer pipe, of the lower end of the inner or next innermost pipe.
A sub-group of top submerged lances has become distinguished by designation as shrouded lances, for which the Outotec Ausmelt TSL technology is well known—see for example, Australian patent 640955 and its counterpart in U.S. Pat. No. 5,251,879 to Floyd. This sub-group is distinguished by the use of a further pipe, external to the typical lance outer pipe. The further pipe comprises a relatively short sleeve or shroud through which the main lance outer pipe extends and which is secured around the upper extent of the outer pipe. The shroud terminates at a location above the molten bath when the lance discharge end is submerged. The discharge of gas down into the reactor space, through a passage between the shroud and the outer pipe, adds to the cooling effect of gas passing through the lance for injection into the slag of the molten bath. The shroud thus assists in maintenance of a sufficient thickness of solidified slag coating on the lower extent of the outer pipe of the lance. The added cooling achievable with a shrouded lance is highly beneficial with a long lance length, particularly if a process with which the lance is used requires a limited flow rate of gas injected by the lance. The cooling effect provided by the shroud is also advantageous when the lance is required to be in operation for a long period. Also, in a furnace operating in the temperature range of about 1100° C. to about 1600° C., the thickness of solidified slag coating on the outer pipe of the lance decreases with increasing temperature. While for a given slag chemistry the amount of super heat generally is not large, use of high temperatures can be dictated by the slag chemistry or end product needs. Thus, added cooling enabled by gas supplied through the shroud becomes increasingly important at high temperatures in ensuring a coating of a sufficient thickness.
A shrouded lance has further important utility. In many instances it is required to supply gas to the reactor space above the molten slag. The gas may be an oxygen-containing gas, such as air or oxygen-enriched air, such as where post-combustion of gases evolved from the bath, or oxidation of evolved metal fume, is required. To serve this purpose the shroud outlet must be positioned correctly relative to the molten bath layer. Too close and any injected oxygen containing gas may interact with the main bath material. Too far away and any post combustion or oxidation reactions may be incomplete. Such reactions can also provide a heat transfer benefit where slag splashed from the bath is heated by these exothermic reactions and so recover some of this energy directly to the bath when the splashed material returns to the main volume of the bath. This ensures that the oxygen potential is controlled in the freeboard such that the slag maintains its conditions while the offgas is oxidized sufficiently to ensure smooth and optimal operation and conditioning of the offgas.
The present invention is directed to providing an improved shrouded lance for top submerged injection.